Wireless communication systems should provide for a large number of secure (or private) communication channels within their allotted frequency space. In order to achieve these goals, spread spectrum systems have been developed. In a spread spectrum type system, spreading codes are used that allow multiple channels to occupy the same frequency range. In order to successfully demodulate a channel, the spreading code and covering code used in connection with the channel must be known. When a demodulation processor is tracking a particular signal path, signal paths associated with other transmitters appear to that processor as noise.
In order to provide for reliable communications, spread spectrum systems typically track multiple signal paths in connection with establishing and maintaining a communication channel between a pair of end points. The different signal paths may result from redundant signals that are provided by additional base stations and base station sectors, or from reflected or multi-path versions of signals. For example, direct sequence code division multiple access (DS-CDMA) communication systems are subject to interference from other DS-CDMA signal sources resulting in signal degradation. This has a deleterious effect on the acquisition, tracking and demodulation of received signals of interest. A system of this type is often described as being interference limited, since strongly interfering signals may create an upper bound on system performance. Therefore, a reduction in interference results in improved processing of the signals of interest and can thus be exploited in terms of increased system capacity, system coverage, data rate or other beneficial system parameters that improve with improved SNR.
Examples of DS-CDMA communication systems include, but are not limited to the forward and reverse links of cdmaOne, cdma2000 or WCDMA. In the forward link of cdma2000, a base station transmits a plurality of signals intended for a plurality of mobile stations. The plurality of transmitted signals includes a pilot channel, a paging channel, a synchronization channel and a plurality of traffic channels. Each traffic channel is encoded, and only mobile stations that know the Walsh covering code associated with a particular traffic channel can decode that particular traffic channel. The plurality of encoded channels is then spread by a pseudo-random-noise (PN) sequence across the system bandwidth. Thus, all mobile units simultaneously share the same forward link frequency spectrum and the traffic channels are distinguished by their assigned Walsh codes and short code offsets. DS-CDMA systems use a high chipping rate for the PN code so that where two multipath signals are separated by more than a chip, the two signals appear approximately independent of each other. Most systems use a RAKE receiver architecture to combine two or more received signals to increase the probability of detection and the ability to correctly demodulate the signal. Each finger independently estimates channel gain and phase using the pilot signal to coherently demodulate different copies of the same signal. The traffic symbol estimates are combined in a combiner to provide a better estimate of the transmitted signal.
Typically, the RAKE receiver fingers or demodulation fingers are assigned to the strongest multipath signals, in order to provide the best possible estimates of the transmitted signals. As the mobile unit changes location or the signal environment changes, the channel will typically change and the signals will change ordering, in terms of power, relative to each other. The changes in the channel are due to various factors, including Rayleigh fading, shadow fading, scattering, diffraction and other physical phenomena that alter the path of the signals. The RAKE receiver fingers combine and demodulate the strongest set of multipath fingers. As the set of fingers changes due to fading and other mechanisms, fingers will be de-assigned and others will be assigned, so that the RAKE receiver contains the signal set that can provide the best possible signal estimates.
The encoded channels broadcast by a base station generally do not interfere with one another due to the orthogonality of the covering Walsh codes and the quasi-orthogonality of the covering quasi-orthogonal function (QOF) codes. However, a DS-CDMA receiver is still subject to two forms of multiple access interference on the forward link. Co-channel interference consists of multipath copies of signal paths that are delayed in time with respect to a signal path of interest. In particular, such signals can cause interference because the orthogonality of the Walsh covering codes is lost whenever a time offset exists between two codes. Specifically, when aligned, Walsh codes form an orthogonal basis, but there may be high cross-correlation when they are not aligned. Cross-channel interference occurs when a combination of transmissions from more than one base station sector or base station are received at the RF front-end simultaneously. Each base station sector is distinguished by a unique PN short code offset. The PN sequence has minimal, but nonzero cross-correlation properties. This manifests itself as cross-correlation interference between signals originating from different base station sectors. As a result, a signal transmitted from another base station that is received at a high power level is capable of masking the signal of interest due to the non-zero cross-correlation interference of the short code and the unaligned Walsh codes.
Methods for removing interfering signal paths from received signal streams have been developed. For example, systems that calculate a projection operator from an interference matrix to suppress interference have been developed or proposed. According to one such system, multiple access interferences are ranked, an interference matrix is formed using only left and right overlapping interference reference vectors related to a symbol of a channel of interest and a projection operator constructed with the interference matrix is applied directly to the reference vector, such that interference is suppressed. The projection operator, which projects the reference signal onto a subspace orthogonal to the interference, involves a matrix inverse. A full or pseudo matrix inverse computation is typically required. However, such prior art systems have been incapable of handling communication systems supporting symbols of different lengths. For example, such interference cancellation systems are unable to cancel interference from signal paths in which the interference consists of symbols having lengths that are less than half the length of symbols contained in a desired signal path, because the interference matrix formed using left and right overlapping interference reference vectors cannot span the longer symbol of interest. Accordingly, such systems cannot be applied to recent spread spectrum communication systems, such as CDMA 2000 that support supplemental (short) Walsh covering codes.
With respect to the need for inverting matrices in the calculation of the projection of the reference signal onto a subspace orthogonal to the interference, various methods, commonly referred to as QR methods, have been developed. The QR methods may be used to decompose a given matrix into an orthonormal basis providing a simple matrix inverse operation. For matrix inversions in a projective subspace apparatus, the normalization step is unnecessary. Therefore, such methods introduce computationally expensive and unnecessary steps. Accordingly, prior art interference cancellation systems requiring matrix inversion computations have been computationally inefficient.